This invention relates to a crane having a boom with hydraulic elevation control. More specifically, this invention is concerned with a crane having a hydraulically controlled boom that is further provided with a load-holding check valve to prevent inadvertent loss of actuating hydraulic pressure during crane operation.
With the increased lifting capacity in many large mobile cranes, a problem has arisen during crane transportation from one job site to another: for many states have enacted highway use restrictions. Often, the restrictions limit the weight that may legally be carried on wheels traversing the state's highways. As a result, some very large cranes must be disassembled into sub-assemblies for transportation.
With some cranes, however, it has been found that a wheeled dolly supporting the end of the boom provides an alternative approach to crane disassembly. In this manner, the actual weight carried by the rear wheels of the mobile crane is reduced by that portion of the boom weight supported by the dolly. Typically, the dolly is attached to the carrier of the mobile crane and is pulled thereby in a tandem fashion.
Mobile cranes are well known which have one or more double-acting hydraulic cylinders or the like to control boom elevation. In more recent times, federal safety regulations have required that the hydraulic boom elevation cylinders be provided with a corresponding load-holding check valve. The load-holding check valve must be connected to the corresponding hydraulic cylinder such that an accidental failure of pressure generating apparatus or hydraulic conduits leading to the hydraulic cylinder will not result in a loss of actuating pressure in the hydraulic cylinder.
The presence of a load-holding check valve in a boom elevation control system effectively prevents the free flow of hydraulic fluid from one end of the double-acting hydraulic cylinder to the other. Accordingly, the boom itself is essentially rigidly fixed to the crane.
This rigid connection creates a substantial problem during transportation of the crane. For example, a relatively small elevational change at the dolly supported end of the boom during travel at highway speeds of, for example 45 miles per hour can induce relatively large impulsive loads and concomitantly high stresses in the hydraulic system.
Such repetitive impulsive loads are deleterious and may cause fatigue problems as well as various other damage and premature failures in affected components. It is, of course, highly desirable to obviate problems of the type noted. Moreover, it would be advantageous to have a means facilitating compliance with highway use statutes which conforms to crane safety requirements while avoiding substantial disassembly of the crane during transportation.
Accordingly, the need continues to exist for an effective means to use a load-holding valve and a hydraulic boom elevation system in a crane while permitting the crane to be readily transported over highways within the allowable statutory weight restriction.